Obesity Epidemic: Overeating Alone to Blame

May 11, 2009 -- Blame the refrigerator rather than the gym for Americans’ ever-expanding waistlines. A new study shows that overeating alone can account for the obesity epidemic in America.

Researchers say that until now no one has quantified the contributions of overeating and decreases in physical activity to the rise in obesity in America since the 1970s.

“There have been a lot of assumptions that both reduced physical activity and increased energy intake have been major drivers of the obesity epidemic,” says researcher Boyd Swinburn, chair of population health and director of the World Health Organization Collaborating Centre for Obesity Prevention at Deakin University in Australia, in a news release.

“This study demonstrates that the weight gain in the American population seems to be virtually all explained by eating more calories. It appears that changes in physical activity played a minimal role.”

Explaining the Obesity Epidemic

The study, presented at ECO 2009 -- The 17th European Congress on Obesity in Amsterdam, Netherlands, used a novel approach to estimate the relative contributions of overeating and physical activity to the obesity epidemic in America.

First, they tested 1,399 adults and 963 children to determine how many calories their bodies burned under normal, real-life situations. Then they calculated how much the adults needed to eat in order to maintain a stable weight, and how much children needed to eat in order to maintain normal growth.

Next, they looked at data from a nationally representative survey (NHANES) that recorded the weight of Americans in the 1970s and early 2000s to determine the actual weight gain over that period. Finally, researchers used national food supply data to analyze how much the participants ate from the 1970s to the early 2000s. Using this information on calorie intake vs. calories burned, they estimated the expected weight gain over the 30-year period based on food intake alone.

Their theory was that if the predicted weight gain based on how much Americans ate was the same as the actual weight gain, overeating alone could explain the weight increase, regardless of any changes in physical activity.

Overeating Largely to Blame

The results showed that the predicted and actual increase in weight gain among children, nearly 9 pounds, matched exactly, indicating overeating was likely to blame

“For adults, we predicted that they would be 10.8 kg (23.8 pounds) heavier, but in fact they were 8.6 kg (20 pounds) heavier,” Swinburn says. “That suggests that excess food intake still explains the weight gain, but that there may have been increases in physical activity over the 30 years that have blunted what would otherwise have been a higher weight gain.”

“To return to the average weights of the 1970s, we would need to reverse the increased food intake of about 350 calories a day for children (about one can of fizzy drink and a small portion of French fries) and 500 calories a day for adults (about one large hamburger),” Swinburn says. “Alternatively, we could achieve similar results by increasing physical activity by about 150 minutes a day of extra walking for children and 110 minutes for adults; but realistically, although a combination of both is needed, the focus would have to be on reducing calorie intake.”

Swinburn emphasized that physical activity should not be ignored as a contributor to reducing obesity and should continue to be promoted because of its many benefits. Nevertheless, from a public policy perspective, expectations regarding what can be achieved with exercise need to be lowered and emphasis should be shifted toward encouraging people to eat less, he says.